In recent years, backplanes over large areas with actively addressed thin-film transistors (TFTs) have found important applications in the production of pixelated devices such as image capturing devices, display devices, and sensor devices. In recent years, the increasingly widespread use of display device alternatives to the cathode ray tube (CRT) has driven the demand for large-area electronic arrays for displays. In particular, active matrix addressed liquid crystal displays are commonly used in lap-top computers and for televisions. However, fabricating such large-area TFT arrays is expensive. A large part of the fabrication cost of the large-area TFT arrays arises from the photolithographic process used to pattern the array. In order to avoid such photolithographic processes, direct marking techniques have been considered as an alternative to photolithography.
Examples of direct marking techniques used in place of photolithography include utilizing a xerographic process to deposit a toner that acts as an etch mask and using an ink-jet printhead to deposit a liquid mask. Both techniques have corresponding problems. Toner-based materials are hard to control and difficult to remove after deposition.
The use of ink-jetted liquids to directly write etch masks is a practical alternative to printed toner although jet printing also possesses inherent complexities. Controlling the feature sizes of printed liquid masks is difficult due to spreading of the liquid on the surface after deposition. For example, when liquid drops are deposited onto a surface, the droplet configuration is largely determined by its wetting properties. Typically, small wetting or contact angles (the angle formed by the edge of a droplet and the substrate surface) are required to obtain good adhesion to a surface but this condition allows the liquid to spread and form relatively large features. On the other hand, if the liquid does not wet the surface due to a high surface energy, a large contact angle will form allowing for the formation of small drop features. However, these printed droplets may adhere poorly. Neither situation is desirable in semiconductor processing—the small contact angle droplets increase feature size while large contact angle droplets give unreliable patterning.
Special piezoelectric ink-jet printheads allow generation of low droplet volumes. Small printed features have been obtained using ink-jet printheads as described in W. S. Wong, et al., “Amorphous silicon thin-film transistors and arrays fabricated by jet printing” in Appl. Phys. Lett., 80, 610 (2002). In the described reference, wax etch masks patterned by ink-jet printing are used to produce feature sizes on the order of 20-40 microns with layer registration to within a few micrometers. However, even with these printheads, the small sizes of features critical to the fabrication of large-area microelectronic arrays have been difficult to achieve. In using a jet-printed feature as an etch mask, the minimum feature size was limited by the smallest droplet, typically in the range of 20 microns.
In many cases, the use of a smaller channel length transistor with a comparatively small gate electrode can improve the performance of a backplane circuit. One possible solution is to use a liftoff process where one uses the ability to print a mask on a substrate where the gaps between the printed mask are small relative to the printed features themselves and then to deposit a metal film over the whole substrate. In this process, the metal film may cover a portion of the printed mask layer and in some cases may cover the printed mask and substrate completely. The mask is then removed chemically (dissolved in solvent), removing the metal above the mask but leaving the metal between the original mask features. Cracks in the metal around the mask allow solvent to attach the mask material through the metal. However, in some manufacturing processes, liftoff may not be the best solution for fabrication of fine features. Removal of the metal in a bath creates a particle-laden solution that can leave particulates on the surface of the substrate reducing yield in some cases.
Thus, a method of forming smaller features, such as small gate electrodes in thin film transistors, using inexpensive printing techniques is needed.